The present invention relates to a friction member and a rotor or stator of an ultrasonic motor.
An example of an ultrasonic motor is shown in FIG. 3.
The cylindrical composite vibrator type ultrasonic motor 14 shown in FIG. 3 is generally composed of a stator 24 comprising a torsional vibrator 22, a longitudinal displacement type piezoelectric element 20, and a stator head 28, and a rotor 10 rotatably coupled thereon.
The torsional vibrator 22 is composed of a laminated piezoelectric member 16 such as lead zirconate titanate and a cylindrical vibrator 18 disposed thereon.
A longitudinal displacement type piezoelectric element 20 which operates as a clutch when actuated and a stator head 28 is disposed on the torsional vibrator 22.
The surface of the stator head 28 which is in contact with the rotor 10 is a stator sliding surface onto which a friction member 30 is applied.
The rotor 10 is bolted with a spring 26 interposed and is thereby urged onto the friction member 30 of the stator 24 at a predetermined pressure (preloaded).
In the composite vibrator type ultrasonic motor 14 having the configuration as described above, an application voltage of the resonance frequency of the torsional vibrator 22 is applied to the torsional vibrator 22 and the longitudinal displacement type piezoelectric element 20. This causes torsional vibration of the torsional vibrator 22 and expansion and contraction of the longitudinal displacement type piezoelectric element 20. The combination of these vibrations allows the rotor 10 to be continuously rotated in one direction.
The driving torque of ultrasonic motors including composite vibrator type ultrasonic motors is largely dependent on the friction force between the stator head 28 (substantially the friction member 30) and the rotor 10. That is, a large driving torque can not be generated unless the friction member has a certain degree of friction coefficient.
In addition, the abrasion resistance of the friction member 30 determines the durability and life of an ultrasonic motor itself.
Accordingly, the friction member is required to be able to withstand for a 10,000-hr driving period and, at the same time, to generate a predetermined driving torque.
In general, a polyimide type resin is used for the friction member 30 but it does not necessarily provide satisfactory results.